1. Field of the Invention
The present invention refers to stirrups and ties for structural members. Such stirrups and ties are used in all the structural members like columns, beams, slabs, footings, piles, chainages, lintels e.t.c. The invention refers also to a method of reinforcement of structural members and to the structural members themselves.
2. Description of the Current Art
It is known that stirrups and ties constitute one of the most critical factors of strength of concrete structural members because they undertake the tensile forces, which cannot be carried by the concrete itself. These tensile forces are due to the shear forces, which load the structural member, and/or to the internal pressure which is created when the structural member is subjected to strong compressive loads.
The usual stirrups and ties of the concrete structural members consist of steel bars of grade 220 MPa up to grade 500 MPa with circular cross-section and diameter from 4 mm up to 14 mm. These stirrups and ties are placed along the structural members at distances from 4 cm up to 35 cm. The longitudinal rebars of the structural members are tied or welded at the corners or at the perimeter of the stirrups and ties. The cross-sections of the structural members take values from the range 15 cm up to 2 m. At the two ends of the bar of every stirrup or tie there are hooks, with length about 10 cm, for the anchorage of the stirrup or tie, which means the transfer of the tensile forces from the steel bar to the concrete.
The main factor of the reliability of the usual stirrups and ties is anchorage. Nowadays there are two types of closed stirrups and ties, which are distinguished by the way of the anchorage of their ends:
The usual stirrups and ties are of simple shape like orthogonal (FIG. 1) or of complex shape (FIG. 1A). The steel bar 11, which is loaded by tension, ends in two hooks 12 which insure the anchorage of the stirrup or tie. The anchorage of the stirrup or tie is improved when the hook of its one end is fastened to the hook of its other end, as it is can been seen for example in the welded connection of FIG. 1. The anchorage of such a stirrup or tie is achieved with hooks at an angle of 90° or more efficiently with hooks at an angle of 135°. The disadvantages of these stirrups and ties are: a) the mechanical anchorage point is very delicate and is a point of probable defective construction, b) the anchorage mechanism confines the use of high-grade steel, i.e. of strength 1200 MPa, in this type of stirrups and ties and the result is that much steel is required with great weight and high cost, c) and there is a need for relatively careful fastening of the two hooks with the longitudinal rebars 10.
Document WO 93/22516 shows grids 40 and grids 60 for the reinforcement of concrete columns and girders respectively. The grids 40 and 60 are formed by longitudinal and transverse members 44, 42 and 64, 62 respectively, which are welded at their intersections leaving a projecting end beyond the weld, at the end of each longitudinal or transverse member. These projecting ends are used to attach the plane grids in order to form the 3-D structure shown in FIG. 4 (see also FIG. 4a).
Document GB 1,086,857 shows a tie (FIG. 2, 5, 7) formed by members, which according to page 1, lines 54 to 57, are “arranged in their desired configuration with their adjacent ends overlapping and secured together by welding leaving projecting ends beyond the weld”.
Document U.S. Pat. No. 4,472,331 discloses a method for building a reinforced concrete structure. When applied for the construction of columns the method employs column reinforcement frames A (FIG. 2) and column shearing reinforcement bands C (FIG. 8). Similarly when applied for the construction of beams the method employs beam reinforcement frames B (FIG. 5) and beam shearing reinforcement bands C (FIG. 9). The problem addressed by the invention of the document is the precise arrangement of the main reinforcement bars of columns and beams (see column 1, lines 25 and 26), and the function of the elements A and B is to receive and position in the bar insertion holes 1 and 1′ the reinforcement bars. Shearing reinforcement of the columns and beams is effected by the elements C and D, which correspond to the stirrups or ties of the invention of the present application. In accordance with D2 the column shearing reinforcement band C and the beam shearing reinforcement band D are “ . . . formed by bending a steel strip into a rectangular frame shape . . . ”. FIGS. 17 and 18 of U.S. Pat. No. 4,472,331 shows the overlapping ends of C and D.
Document FR 532.620 discloses reinforcement for beams or columns comprising ties 8 encircling the longitudinal rebars. The document discloses that the ties 8 have an annular shape (“forme d'anneaux”, page 2, line 40) and does not give any information on their ends other that on page 2, lines 42 to 44. In accordance thereto the ties form a “double crochets”, which is shown in the cross-section presented in FIG. 5. FR 532.620 discloses a particular reinforcement that includes ties of FIGS. 4 and 6. These are elongated ties that may receive two parallel main reinforcement bars.
Further spiral stirrups of simple or complex shape are shown in FIG. 2. These stirrups and ties consist of a steel bar 13, whose unwinding in space creates a stirrup cage with no discontinuation. Their two ends 15 achieve the anchorage of such stirrups and ties in concrete. These stirrups and ties, compared with the usual stirrups and ties, improve the anchorage and permit the use of high grade steel but they have two major disadvantages: a) unstable anchorage of the first and the last helix, b) and excessive weight which makes the use of the spirals difficult during the reinforcement of the structural member.
The object of the invention is to propose stirrups and ties that do not have the disadvantages of the known stirrups and ties. An object of the invention is to develop stirrups and ties that do not have the known problems of anchorage of the usual stirrups and ties. Further objects of the invention is a) the development of a method for the placing of the whole reinforcement in concrete with no anchorage problems, and b) the provision of structural members with high ductility and ability to withstand high seismic loads.